The present invention refers to an apparatus for depositing particulate or powder-like material on the surface of a substrate, said apparatus comprising a hollow tube-like chamber consisting of a heat-resistant material, said tube-like chamber having a first and a second end, means for supplying solid particulate or powder-like material into said tube-like chamber at said first end, means for supplying a plasma gas into said tube-like chamber at said first end, and means for generating a plasma torch in the interior of said hollow tube-like chamber including ignition means for the ignition of said plasma torch.
In order to provide the surface of a substrate with a coating of particulate or powder-like material, so-called plasma coating devices are known in the art for several years. Thereby, in such devices, a metallic or ceramic powder material is blown into a plasma torch and, therein, heated up to a very high temperature. The powder material is molten and accelerated in the plasma torch and is finally thrown towards the surface of the substrate to be coated with very high speed.
There is a first category of industrially used plasma torch coating devices. These devices are powered with direct current whereby there is provided a pair of electrodes connected to a direct current power supply. A plasma gas jet is blown into the space between the two electrodes and, after an initial ignition process, is ionized therebetween in order to provide a plasma torch. The temperatures in the interior of the plasma torch are very high, i.e. sufficiently high to melt a particulate or powder-like metallic or ceramic coating material which is blown into said plasma torch.
A common characteristic of these direct current plasma coating devices is that the particulate or powder-like material is blown into the plasma torch, that it is thereby molten and that it is accelerated to a relatively high speed before it hits the surface of the substrate to be coated. The maximal speed and the maximal temperature of the particulate or powder-like coating material thereby depends on the characteristics of the plasma torch. The direct current plasma coating devices known in the art are in a position to provide an escape speed of the coating material in the region of about 100 to 300 m/s; however, the outlet cross section is relatively small. The consequence is that a long time is required to coat the surface of a large substrate. Thus, such devices are more preferred for the coating of small substrates.
Another possibility for the generation of a plasma torch is to inductively supply power to the plasma gas in order to create a high energy plasma torch, of course after an initial ignition process. For this purpose, there is provided a hollow tube-like chamber consisting of a heat-resistant material, said tube-like chamber having a first and a second end, means for supplying solid particulate or powder-like material into the tube-like chamber at said first end, means for supplying a plasma gas into said tube-like chamber at said first end, and an induction coil powered by a high frequency generator located at said second end for generating a plasma torch in the interior of the hollow tube-like chamber. Such an apparatus, however, is not in a position to provide a particularly high escape speed of the molten particulate coating material, but the usable outlet cross section, compared with a direct current plasma torch coating device as discussed hereinabove, is much greater. Thus, such a inductively coupled plasma coating device is much better suited for the coating of the surface of large substrates.
The U.S. Pat. No. 4,853,250 discloses a method of depositing particulate material on a substrate. The apparatus used therefor is based on a conventional inductively coupled plasma torch coating device. However, the disclosure of the above mentioned Patent shows a way to increase the escape velocitiy of the particulate material molten in the plasma torch, as compared to similar devices known in the art. It is known that usual inductively coupled plasma coating devices can reach an escape velocity of approximately 10 to 30 m/s, while the apparatus described in the above mentioned Patent enables the molten coating material with a velocity of approximately 100 m/s or even more.
A great disadvantage, however, is common to both the conventional inductively coupled plasma coating devices and to the device according to the above mentioned Patent: The temperature distribution over the cross-section of the apparatus, particularly over the cross section of the hollow tube-like chamber in which the plasma torch is present and which is initiated and maintained by means of an induction coil, is not very uniform. With other words: In the region of a wall of the hollow tube-like chamber, the temperature of the plasma torch is substantially higher, continuously decreases towards the center of the tube-like chamber and, thereafter, increases again towards the opposite chamber of the tube-like chamber.
It is understood that these characteristics are not desired because the uniformity of the coating of the substrate is impaired and because the time required to coat a substrate of considerable size is increased.
A further disadvantage of an apparatus of the kind discussed hereinabove, i.e. inductively coupled plasma torch coating devices, is that the high frequency generator unit required to power the induction coil has to provide a relatively high amount of energy to maintain a plasma torch having the desired shape and intensity. It is known to every person skilled in the art that a device of the aforementioned kind requires a power supply which must be able to provide an energy in the region of 25 to 150 kW with a frequency of some MHz; thus, it is important to design the real coating device, i.e. the heat-resistant tube-like member with the integrated induction coil as well as the high frequency generator, either as a single unit or to take measures that the high frequency generator is not too far from the real coating device because the transmission of high energy in the region of several dozens of kW at a frequency of several MHz poses severe problems.